Tonometers for measuring IOP were originally developed as “contact” type instruments, meaning that a portion of the instrument is brought into contact with the cornea during the measurement procedure. A well-known instrument of this type is the Goldmann applanation tonometer (GAT) originally developed during the 1950s. The GAT measures the force required to flatten (“applanate”) a known area of the cornea, and is used today as a standard against which other types of tonometers are compared to assess measurement accuracy.
Patient discomfort caused by contact tonometers such as the GAT led to the development of “non-contact” tonometers (NCTs) which operate by directing an air pulse at the patient's cornea to cause applanation. As the cornea is deformed by the fluid pulse, an opto-electronic system monitors the cornea by detecting corneally reflected light from a beam obliquely incident upon the cornea, and a peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat.
In state of the art NCTs, a pressure transducer measures the pump plenum pressure as the pulse is generated to provide a plenum pressure signal, whereby the plenum pressure at the moment applanation is achieved (indicated by a sharp peak in the applanation signal) can be determined. The plenum pressure at applanation is then converted to an IOP value in units of millimeters mercury (mmHg) using a linear regression equation stored during instrument clinical calibration relative to GAT as a reference. A primary index of an NCT's reliability is the standard deviation of differences Sd of matched pairs of NCT and GAT clinical readings.
Current NCTs provide reasonably reliable IOP measurements, however recent studies indicate that corneal effects can have a significant impact on conventional NCT readings. This is not surprising, given that the cornea must be acted upon during the pressure measurement process and the air pulse must expend some of its energy “bending” the corneal tissue itself. Consequently, IOP readings are falsely inflated to a degree that varies from patient to patient depending upon the physical properties and characteristics of the patient's cornea at the time of measurement, such as thickness, hydration, and intrinsic tissue properties. True IOP is independent of the properties of the cornea.
During a non-contact IOP measurement, the cornea is deformed from its original convex state through a first state of applanation to a slightly concave state, and is allowed to return from concavity through a second state of applanation to convexity as the air pulse decays. Indeed, a second peak corresponding to the second state of applanation is known to occur in an applanation signal. U.S. Pat. No. 6,419,631 describes a non-contact tonometry method in which both a first plenum pressure P1 and a second plenum pressure P2 are used to calculate IOP. In one disclosed embodiment, a first IOP value (IOP1) is calculated by inputting P1 to a stored regression equation derived from a plot of clinical GAT measurements against P1 measurements for a population of eyes, a second IOP value (IOP2) is calculated by inputting P2 to another stored regression equation derived from a plot of clinical GAT measurements against P2 measurements for a population of eyes, and the two IOP values (IOP1 and IOP2) are averaged to yield a final result. In another disclosed embodiment, the pressures P1 and P2 are averaged and the result is input to a single stored regression equation derived from a plot of clinical GAT measurements against the average of P1 and P2 as measured for a population of eyes. A recent study of pre- and post-LASIK eyes demonstrates an unexpectedly large change in IOP after LASIK surgery when IOP is calculated according to this method, evidence that corneal effects continue to influence IOP measurement.
U.S. Pat. No. 6,817,981 describes a measurement approach intended to identify eyes having above-normal intraocular pressure. Under this approach, a standard NCT measurement of IOP is made and corneal hysteresis associated with the IOP measurement is found by calculating a pressure difference between the inward applanation pressure P1 and the outward applanation pressure P2. The standard IOP measurement value and the corneal hysteresis define a point in two-dimensional space. The location of the point is then compared to a normality line or curve in the two-dimensional space defined by data points from a statistically significant population of eyes, whereby a difference of the measured IOP from an expected IOP for a given corneal hysteresis value can be observed.
The PASCAL® Dynamic Contour Tonometer developed by SMT Swiss Microtechnology AG is a contact tonometer designed to measure true IOP. The PASCAL® tonometer comprises a concave sensor tip intended to match the convex contour of the cornea. The sensor tip is spring loaded to provide constant appositional force to the cornea, and includes a solid state piezoresistive pressure sensor for continuously measuring IOP.